storage: Need to clear pool prior to refreshPool during Autostart

https://bugzilla.redhat.com/show_bug.cgi?id=1176510

When storageDriverAutostart is called path virStateReload via a 'service
libvirtd reload', then because the volume list in the pool wasn't cleared
prior to the call, each volume would be listed multiple times (as many
times as we reload). I believe the issue would be introduced by commit
id '9e093f0b' at least for the libvirtd reload path, although I suppose
the introduction of virStateReload (commit id '70da0494') could be a
different cause.

Thus like other places prior to calling refreshPool, we need to call
virStoragePoolObjClearVols

(cherry picked from commit 1d2e4d8c)

src/storage/storage_driver.c

@@ -125,6 +125,7 @@ storageDriverAutostart(virStorageDriverStatePtr driver)
         }
 
         if (started) {
+            virStoragePoolObjClearVols(pool);
             if (backend->refreshPool(conn, pool) < 0) {
                 virErrorPtr err = virGetLastError();
                 if (backend->stopPool)